It is well-known that ceramic materials are much stronger in compression than in tension. Therefore, "tempered" glass is typically used for glass doors, vehicle glazings, and other high-strength-requirement applications. Residual compressive stresses are intentionally induced in tempered glass by heating a glass sheet to a temperature near its softening point, removing it from the heating furnace, and quickly directing blasts of a cooling fluid, such as air, toward the major surfaces of the glass sheet. The surface regions of the glass sheet contract because of the drop in temperature as a result of convective heat transfer to the cooling air. Thus, the major surface regions of the glass sheet become rigid, while the central portion of the glass sheet remains hot and can adjust its dimensions to the surface region contractions. When the central region of the glass sheet cools and contracts slightly at a later time, compressive stresses are produced in the major surface regions of the glass sheet. A constant cooling rate applied to both major surfaces of the glass sheet, resulting from an identical flow of constant-temperature cooling air to both major surfaces, theoretically would produce a parabolic stress distribution when measured normal to the major surfaces of the glass sheet.
Tempered glass is particularly useful for high-strength-requirement applications because the exposed surfaces of the tempered glass sheet are under residual compressive stress. Glass failure usually occurs under an applied tensile (rather than compressive) stress. Therefore, since failure in a glass sheet almost always is initiated at one of its major surfaces, e.g., by striking the glass sheet, any applied stress must first overcome the residual compression near the surface of the glass sheet before that region is brought into tension such that failure may occur.
It is generally known in the art of manufacturing automotive glazings to heat a glass sheet templet to a temperature above its plastic set temperature, usually about 1200.degree. F., then form the templet to a desired curvature by either gravity forming or press bending the hot glass, and thereafter temper the formed glass templet by directing streams of a tempering fluid, usually moist air, against the major surfaces thereof. During the tempering operation, it is known to support the formed glass sheet on a support ring, comprising a rigid structure conforming generally in outline and elevation to the underside peripheral marginal surface of the formed glass sheet.
During the tempering operation, the blasts of tempering fluid rapidly cool the major surfaces of the formed glass sheet in all areas, except those areas near points of contact between the tempering support ring and the underside peripheral marginal surface of the glass sheet. In those areas, cooling is retarded due to the restricted flow of tempering fluid caused by interference with the tempering support ring. Thus, those areas of the ultimately produced glass sheet may be stressed in tension while the majority of the major surface area of the glass sheet is stressed in compression. This stress imbalance often leads to spontaneous breakage of the glass sheet during use in a motor vehicle.
Moreover, other variables in the tempering process can result in poor quality, nonuniform tempering, wherein the configuration of the actual stress distribution measured across the thickness of the glass sheet at any Point along the surface of the glass sheet varies markedly from an idealized parabola. Clearly, the quality of temper induced into a glass sheet is difficult to control, and even more difficult to measure quantitatively.
It would be desirable to devise a method for determining the quality of the temper induced into a glass sheet. Such a method should be simple, and yet give consistent, reproducible results which could be used to control the tempering process and therefore the quality of the tempered glass sheet produced thereby.